An a.c. cable, for example, is described in German Patent No. 38 11 051.
The development of a.c. cables with electric conductors containing superconducting metal oxide compounds as conducting materials is of particular interest today. Such superconducting metal oxide compounds with high critical temperatures T.sub.c of preferably over 77 K, which can therefore be cooled with liquid nitrogen at normal pressure, are generally known. These compounds are referred to as high-T.sub.c or high-temperature superconducting materials (abbreviated as HTSC materials). Suitable metal oxide compounds include in particular cuprates, for example, Y--Ba--Cu--O or (Bi,Pb)--Sr--Ca--Cu--O system-based cuprates.
Conductors allowing superconducting cable cores of a.c. cables to be built can be constructed from these HTSC materials for electric power transmission with low losses and small cross sections. Economic advantages are thus achieved in comparison with known normally conducting cables, since the a.c. field losses, including the energy consumed in a refrigeration system used to dissipate said losses, are lower than the losses in a comparable normally conducting cable.
Estimates and loss measurements on cable models lead to the expectation that this object cannot be easily achieved if cable cores with a plurality of layers of strip-shaped HTSC elementary conductors, for example, are required for the current-carrying capacity in question. It is considered that this is due to the movement, accompanied by losses, of the magnetic self-field flux into and out of the superconductor and induced eddy currents in metallic conductor components. According to a known, empirically supported loss theory, the magnetic field on the conductor surface should be selected to be as small as possible. This theory, however, applied to an a.c. cable, means that the diameter of the cable cores should be sufficiently large so that the current can be carried with a single layer of single superconductors. This, however, causes problems regarding
low flexibility and large allowable radii during manufacturing, transportation, and storage, PA1 a high volume of electrical insulation, high dielectric losses, and high capacitance, PA1 large cryogenic shell surface and considerable influx of heat into the coolant. PA1 in the external space around the conductor layer (with radius r&gt;R), the field is purely azimuthal: PA1 H.sub..psi. =J.sub.z R/r=J cos.alpha.R/r. PA1 in the internal space enclosed by the conductor layer (with radius r&lt;R), the field is homogeneous and is directed along the cable axis z: PA1 H.sub.z =J.sub..psi.=Jsin.alpha., PA1 .alpha. is the wire angle between the individual superconductors and the cable axis (in the z direction), PA1 R is the radius of the single conductor layer, PA1 J=I/(2.pi.Rcos.alpha.) is the specific current density on the conductor surface (=the current in a strip-shaped single conductor per unit of strip width).
No measures are currently known to eliminate these problems.